For decades, coaches, doctors, and athletes have trumpeted the myth that squatting below parallel is bad for your knees. Go into a commercial gym and do a deep squat with anything more than the bar on your back and members around you will cringe and tell you about how squatting is the reason they hurt their knees, back, etc. back in the day. The truth is, beyond these old wives tales, there’s virtually ZERO evidence that squatting below parallel, assuming you’re doing it correctly, is bad for you. In fact, the strength gained as a direct result of squatting can do wonders for protecting the knee from injury.

This myth can be traced back to the 1960s and University of Texas researcher Dr. Karl Klein. In a 1961 study, Klein tested the knee laxity of 128 weightlifters that regularly did full squats and 360 college students that did not. He found that the knee joints of weightlifters showed greater laxity and instability compared to control subjects and concluded that the full squat “should be discouraged from the standpoint of its debilitative effect on the ligamental structures of the knee." (Klein 1961) In August of the same year, the American Medical Association agreed with Klein, citing "potential for severe injury (medial cartilage deterioration) to the internal and supporting structures of the knee joint." (Shea 2009) The setup of Klein’s study was extremely flawed, though. Bill Starr commented on this in his book, The Strong Shall Survive: Strength Training for Football.

“One, the applying of the pressure was far too subjective. The tester could push harder and harder on the knee joint and secure a pre-determined reading. Many athletes complained that the tester actually hurt their knees because he pushed so hard. Did he push equally hard on each subject? Two, the tester always had the subject classified as a squatter or non-squatter before he tested him, rather than after. This certainly gave the tester the opportunity to have a built-in prejudice... By applying the same testing procedure, another researcher, with an eye to reversing the findings, could quite easily do so. A pure piece of research should not have this wide margin of testing subjectivity unless it is specifically spelled out in the conclusion, which it definitely was not in this case.” (Starr 1976, 39-40)

“Dr. Karl Klein’s study at the University of Texas in 1961 is a poorly designed and badly conducted mess that has never been replicated and has been successfully rebutted many times. Klein concluded that below parallel squats produced “loose” knees, although no other training protocol was evaluated for comparison, no other tester administered the measurements, and all the data was biased by pre-test questioning of the subjects.” (Rippetoe 2007, 66-67)

As Rippetoe said, several researchers have disproven Klein’s findings since the ‘60s. In one study by Chandler et al. (1989), male powerlifters and weightlifters were tested against a control group of untrained subjects and showed significantly tighter (more stable) knee joint capsules in anterior drawer tests.

Furthermore, numerous studies have concluded that stopping above parallel in a squat produces higher shearing forces on the knee than going all the way down (Li 2004, Markolf 1996). This is due to the increased stretch on the hamstrings when the lifter descends all the way to the bottom. Below parallel, the tension on the hamstrings produces a force on the back of the tibia that helps to counteract the force of the quadriceps on the front. This balancing of forces helps stabilize the knee and takes stress off of ligaments like the ACL.

Generally, the worst thing that will happen as a result of excessive squatting is you’ll develop some patellar tendonitis, but that’s manageable with a little ice, rest, and ibuprofen. In 1999, The Journal of Athletic Training published a study on injuries among elite weightlifters. Injury reports were gathered from the United States Olympic Training Centers over a six-year period. 85% of all reported knee injuries were tendonitis and the recommended time off from training for most injuries (90.5%) was one day or less (Calhoon, 1999).

In 1991, the National Strength and Conditioning Association (NSCA) conducted a review of literature and published a position paper on the squat. Among other things, they state:

Squats, when performed correctly and with appropriate supervision, are not only safe, but may be a significant deterrent to knee injuries.

Excessive training, overuse injuries, and fatigue-related problems do occur with squats. The likelihood of such injuries and problems is substantially diminished by adherence to established principles of exercise program design.

The squat exercise is not detrimental to knee joint stability when performed correctly.

Injuries attributed to the squat may result not from the exercise itself, but from improper technique, pre-existing structural abnormalities, other physical activities, fatigue, or excessive training.

Funny, the NSCA seems to think squatting is safe unless you do it incorrectly. The same can be said for crossing the street. As a general rule of thumb, doing things differently than they’re meant to be done will mess you up.

The reason Klein’s work was so welcomed 50 years ago, and why his conclusions are still accepted by some people today, is because most people don’t like squatting deep, and Klein gave them an excuse not to do it. Squatting is hard. It’s uncomfortable; and many people choose to avoid discomfort, regardless of the results it can produce. Saying deep squats are dangerous makes these people feel better about the fact that they can’t, or don’t want to, do something difficult. The bottom line is if you’re interested in improving athletic performance squats should be a staple of your program.

Klein, Karl, 1961, “The Deep Squat Exercise as Utilized in Weight Training for Athletics and its Effect on the Ligaments of the Knee,” Journal of the Association of Physical and Mental Rehabilitation 15: 10.